The well recognized difficulties with powdered dyes have resulted in efforts to formulate various classes of dyes in liquid form. One of the strongest impetuses to this desired for liquid formulations has been the advent of automated metering and handling equipment which greatly reduces the labor required and the potential for error in utilizing dyestuffs in practical applications. Another driving force has been the desire on the part of dyestuff manufacturers to avoid the energy intensive step of drying the wet filter cake which usually results upon isolation of the synthesis product.
Proposals for liquid dye formulations have been made with regard to a number of diverse types of dyes. One of the earliest commercially successful liquid formulations occurred in the field of paper dyes. One of the earliest of these formulations is described in U.S. Pat. No. 3,905,949 and is still on the market today. The approach utilized with these paper dyes was to render the dyestuff molecule sufficiently soluble in water to be formulated in concentrated aqueous solutions. However, this approach could not be readily extended to all types of dyes particularly those which could not be rendered sufficiently soluble to permit the preparation of commercially significant concentration or those which had a tendency to react with water such as the fiber reactive cotton dyes. In these cases it has been proposed to prepare suspensions or dispersions in water.
There has been no commercially successful aqueous dispersion or suspension of anionic acid dyes marketed in the United States. An early proposal described in U.S. Pat. No. 3,770,371 which related to fiber reactive dyes and metallizable dyes as well as acid dyes involved the addition of sufficient salt to the aqueous suspending media to render the dyes of interest insoluble in combination with the addition of a "stabilizing agent" which was either an anionic or non-ionogenic dispersing agent or surfactant. It appears that the dyestuff was either used as obtained from the filter cake or was subject to very extensive size reduction such as grinding for thirty hours in a glass bead mill. The amount of "stabilizing agent" utilized was evidently dependent upon whether or not the dyestuff was subject to size reduction with stabilizing agent to dyestuff ratios in excess of 20:1 being utilized with the unmilled presscake in contrast to the ratio of 4:1 utilized with the glass bead mill dyestuff formulations. The amount of salt utilized, however, appears to be independent of whether or not the dyestuff was size reduced and independent of the amount of stabilizing agent utilized. The ratio of dyestuff to salt ranged between 1.2 and 2.6. Later workers from the same company, Ciba-Geigy, characterized this approach in U.S. Pat. No. 4,110,073 as being unsatisfactory because "the colloidal-chemical stability of the dispersion is, however, consequently reduced and unstable flocculant dispersions are obtained". They proposed instead uniform extensive size reduction of the dyestuff particles in combination with "anion active dispersing agents". This proposal was reported to have resulted in the formation of fairly concentrated aqueous dispersions which were stable for as long as six months. In particular, these dispersions were reported not to sediment and to maintain their original viscosities after six months room temperature storage. These viscosities were reported to be in the range of between 3 and 500 centipoise, but from the working examples it is evident that this viscosity was measured at infinite shear. In particular, these viscosities were determined in accordance with the technique of "Casson" which is reported in the literature as involving an extrapolation from several measured viscosities at different shear rates to what the viscosity would theoretically be at infinite shear. This technique is described in "Fundamentals of the Rheology of Pigment Dispersions" by W. K. Asbeck appearing at page 65 et seq. of the January, 1961 issue of the Official Digest, as well as in the "Measurement of Rheology Thixotropic Organic Coatings and Resins with a Brookfield Viscometer" by Perry E. Pierce appearing at page 35 et seq. of the June, 1971 issue of the Journal of Paint Technology.
Dyestuff dispersions prepared in accordance with the U.S. Pat. No. 4,110,073 appear to have limited commercial utility because they have been found to have very high viscosities at lower shear rates. The market is evidently unwilling to accept a product with a low shear viscosity in excess of 10,000 centipoise because of its appearance and difficulties in stirring, pumping, pouring and otherwise handling the product. This high viscosity is evidently the result of both the very small dyestuff particle size and the high surfactant content with dyestuff to surfactant ratios between 5 and 1.75 being utilized.
Certain other proposals have been made with regard to certain other specific classes of dyestuffs. German Offenlegungsschrift No. 2,843,015 proposes the preparation of aqueous dispersions of the more water-soluble fiber reactive dyestuffs by stirring presscake and salt into the water. No dispersant or dyestuff particle size reduction is utilized. On the other hand, U.S. Pat. No. 4,264,323 teaches the preparation of aqueous concentrates in which fairly soluble fiber reactive dyes are subject to substantial size reduction and then added to water containing a high level of an anionic dispersing agent. United Kingdom Pat. No. 1,001,182 describes the formation of a "micro-crystalline suspension" utilizing a metallizable dye ground to a particle size between 2 and 3 microns and approximately 20% of a surfactant based on the weight of the dye. No information is given with regard to the stability of this suspension or its viscosity.